North Atlantic Oscillation (NAO)

The North Atlantic Oscillation (NAO) index data provided here is the normalized pressure difference between Gibraltar and SW Iceland (Reykjavik) updated from Jones et al. (1997). Click the 'About' tab for more information.

The standard NAO index data file has been provided in the same format since 1998. This was supplemented by a version stored with 3 decimal places in 2017. The extra precision is provided for users who are making further calculations where rounding to the appropriate precision is best done at the end of the calculations.

Details of how CRU circulation indices are calculated is available here.

for year = 1821 to endyear
  format(i5,12f7.2,f8.2) year, 12 * monthly value, annual value

for year = 1821 to endyear
  format(i5,12f8.3,f8.3) year, 12 * monthly value, annual value

format(i1,i5,i5,a) ?,startyear,endyear,name
  for year = startyear to endyear
    format(i5,12i6) year, 12 * monthly value

Change log

• 2018-01-02: New visualisations and recent values pages added
• 2017-02-08: Some earlier values have been corrected following a comparison of source datasets, and additional NAO data file with 3 decimal places
• 2004-03-10: Correction to the stated data format
• 2000-12-10: The values for Gibraltar have been altered to correct an error

The North Atlantic Oscillation (NAO) is one of the major modes of variability of the Northern Hemisphere atmosphere (Hurrell, 1995). It is particularly important in winter, when it exerts a strong control on the climate of the Northern Hemisphere (Osborn, 2011). This is also the season that exhibits strong interdecadal variability (Osborn, 2004). For winter, the difference between the normalised sea level pressure over Gibraltar and the normalised sea level pressure over Southwest Iceland is a useful index of the strength of the NAO. Jones et al. (1997) used early instrumental data to extend this index back to 1823.

The NAO is traditionally defined as the normalized pressure difference between a station on the Azores and one on Iceland. An extended version of the index can be derived for the winter half of the year by using a station in the southwestern part of the Iberian Peninsula (Hurrell, 1995). Here we give data for SW Iceland (Reykjavik), Gibraltar and Ponta Delgada (Azores). The NAO calculated from Gibraltar and SW Iceland is also given (Jones et al., 1997).

Details of how CRU circulation indices are calculated is available here.

Notes about these visualisations of the NAO index:

1. They all use the Jones/CRU NAO index (Gibraltar minus Iceland). This is most useful during the winter half of the year when Gibraltar and Iceland sea level pressures (SLP) are strongly anti-correlated and the NAO index measures a coherent pattern of stronger/weaker atmospheric flow into NW Euope. Results are also shown for the summer, but these should be interpreted carefully due to more complex circulation regimes in summer that may not be well-described by this NAO index, not least because Gibraltar SLP is influenced by the Iberian thermal low during summer months.
2. The index was generated using a 1951-1980 reference period and therefore has a mean of zero during this period. It is inconvenient that this is a period of anomalous NAO index for most seasons, so the long-term mean of the index is not very close to zero when this reference period is used. Therefore, timeseries are also shown after expressing as anomalies from their 1901-2000 means, a longer and more representative baseline. Note that the index itself was still derived using the means and standard deviations of Gibraltar and Iceland SLP from the 1951-1980 reference period, but the index timeseries is subsequently re-anomalised relative to 1901-2000.

• When using the NAO data, cite "Updated from Jones et al. (1997)"
• When using these NAO visualisations, cite "Updated from Osborn (2011)"

Winter DJFM

Winter DJF

Spring MAM

Summer JJA

Autumn SON

Cold Season ONDJFM

Warm Season AMJJAS

Annual Jan to Dec

Winter DJFM

Winter DJF

Spring MAM

Summer JJA

Autumn SON

Cold Season ONDJFM

Warm Season AMJJAS

Annual Jan to Dec

Recent and Highest/Lowest Ranked NAO Values

Notes about these NAO index values:

1. These values are all from the Jones/CRU NAO index (Gibraltar minus Iceland). This is most useful during the winter half of the year when Gibraltar and Iceland sea level pressures (SLP) are strongly anti-correlated and the NAO index measures a coherent pattern of stronger/weaker atmospheric flow into NW Euope. Results are also shown for the summer, but these should be interpreted carefully due to more complex circulation regimes in summer that may not be well-described by this NAO index, not least because Gibraltar SLP is influenced by the Iberian thermal low during summer months.
2. The index was generated using a 1951-1980 reference period and therefore has a mean of zero during this period. It is inconvenient that this is a period of anomalous NAO index for most seasons, so the long-term mean of the index is not very close to zero when this reference period is used. See NAO Visualisations tab to see a more representative baseline.